It's finally happening. Nick has his hands around Amy's throat. He has her on the ground and she's struggling to fight back. It's his ultimate fantasy, the thing he's been imagining he'd do to her since he figured out she was framing him—but in the end, he just can't do it. Killing Amy would turn him into the monster that everyone thought he was when they suspected him of murder, and he can't go back to that.
The truth is, Amy's right—life without her would be impossible for him. She indulges the part of him that's still the spoiled, favorite child, even though her behavior is dangerous and completely insane. Amy is a toxic parasite, but Nick needs her in order to survive. He decides not to kill her. He just decides to stop her once and for all.